Conventional traction elevators include a motor, for moving the car between floors, a solid state elevator drive that dictates the speed and direction of rotation of the motor, and a car logic controller that controls the drive responsive to various elevator operating conditions, such as the activation of car and hall call buttons, the position of the doors, the activation of safeties and, in multiple car elevator banks, commands from the group supervisory control. When responding to a hall or car call, one of the functions of the controller is to generate speed control signals, based on a predetermined acceleration and deceleration speed profile, to move the car quickly and smoothly to the target floor. The speed control signals are fed to the elevator drive which, in turn, produces an appropriate voltage and current output such that the motor rotates at the dictated speed.
During a run between floors, the controller generates the velocity command profile, which may be either time-based or position-based, as a function of instantaneous elevator position and velocity, which are calculated based upon signals from a position encoder mounted on the speed governor. The profile computation takes place in a central processing unit ("CPU"), which sends speed command signals to a speed control computer card containing a digital signal processor ("DSP"). The DSP, in turn, produces speed command signals and sends such signals to the solid state elevator drive, for example an MG, SCR, or variable voltage/variable frequency (VVVF) drive.
Elevators are provided with one or more backup systems to stop the car at the upper and lower ends of the hoistway in the event that the normal speed control signals would fail to do so. One such system is known as the Normal Terminal Stopping Device (NTSD), which is designed to slow down and stop the car at the upper and lower terminal landings when it senses that the normal speed control will overrun the top or bottom floor. For example, if the CPU receives a faulty position encoder signal, the CPU may determine that the car is further away from the terminal than is actually the case, and generate speed signals that, if followed, would carry the car beyond the terminal landing. Should this occur, the NTSD system is designed to override the normal speed signals and bring the car to a stop at the terminal. An NTSD system is required by the ASME ANSI A17.1 Safety Code For Elevators, as well as by various local jurisdictions.
The car is expected to remain in service following an NTSD slowdown and stop, as contrasted with a more drastic emergency stopping device that shuts down a car and keeps it out of service. Thus, the NTSD terminal slowdown pattern must be relatively smooth. Also, it is desirable that the NTSD system should not override the normal control means as long as the CPU-generated speed control signals remain within a certain acceptable range of the correct values. For these reasons, NTSD equipment is designed to provide a backup slowdown pattern similar in profile to the normal slowdown pattern, but that allows some margin of error beyond the normal slowdown pattern generated by the CPU.
In order to be a reliable backup to the normal control system, the NTSD system needs to be independent of the normal control means for stopping the elevator at the terminal. Therefore, while the CPU dictates speed control signals based upon position encoder signals, the NTSD system is based on a table of speed values which are stored separate from the normal speed control signals, and is controlled responsive to vanes which are located in the hoistway, rather than the position encoder, to provide independent verification of actual elevator car position.
In known NTSD systems, a plurality of metal vanes are positioned near the top and bottom of the hoistway, at predetermined distances from the terminal landings, defining a zone within which a terminal slowdown and stop must occur. Each vane is encoded with a series of identifying holes, which are read by an optical sensor on the car. The vanes form a series of fixed checkpoints representing actual elevator position. NTSD speed values are set during initial elevator installation, and may be re-set during subsequent elevator servicing. To set NTSD values, a normal high speed run is conducted into the terminal landings. As the car passes each vane, the CPU calculates an NTSD value based upon the normal speed control value plus some margin, as described further below.
Thereafter, during normal elevator operation, as the car passes each NTSD vane, the DSP fetches the NTSD speed from a lookup table, and generates a time based speed profile curve having a predetermined deceleration rate, which is greater than the normal deceleration rate. More particularly, as shown in FIG. 1, which is a plot of dictated speed versus time, the speed values derived from the NTSD lookup table produce a stepped profile. A smoothing filter, however, produces an NTSD pattern based on an interpolated speed profile, which decreases linearly until the speed value has reached the NTSD speed of the next vane. The NTSD speed will remain constant until the car reaches the next vane, whereafter the NTSD speed will again start to decrease, at the predetermined deceleration rate, until the NTSD speed for the subsequent vane is reached. The NTSD system is designed so that the NTSD speed reaches the velocity for the next vane prior to the time the car would reach the next vane under normal conditions.
Each time a speed signal is received from the CPU, the DSP compares the dictated signal with the corresponding NTSD speed, taken from the interpolated speed profile curve, and outputs the lower of the two values as a speed control signal to the motor control static drive. Thus, if the speed value requested by the CPU is higher than the NTSD value, the NTSD system "clamps" the speed at the NTSD limit.
If the speed signal from the CPU exceeds the NTSD speed value, it means that the car is travelling too fast to be stopped using the normal deceleration profile. As a result, the deceleration slope of the NTSD pattern must be steeper than the normal deceleration pattern in order to prevent the car from overshooting the terminal. The existing NTSD pattern is therefore both a certain amount greater than the normal pattern (to allow a margin of error), and has a steeper deceleration slope. A conventional design is based on NTSD default values at each vane which are 4% plus 15 fpm above the normal speed values. Between vanes, the NTSD pattern has a deceleration slope which is 10% greater than the normal pattern deceleration slope. All three of these parameters are adjustable to use values other than the defaults.
There are a number of drawbacks with conventional NTSD systems, which complicate the adjustment of the system for proper operation. Examples will be discussed in connection with FIGS. 2-5.
First, jobs that use a reduced-stroke buffer employ an Emergency Terminal Speed Limiting device (ETSL). The ETSL device is activated in the event that the car is approaching the upper or lower terminal landing, and neither the normal speed control nor the NTSD system have slowed the car sufficiently to stop at the landing.
As shown in FIG. 2, there is a time lag between when the controller dictates a speed and when the motor actually reaches such speed. Therefore, during deceleration the actual motor speed will be higher, at any given moment, than dictated speed. Although NTSD dictated speed is substantially less than the ETSL limit, the margin between actual car speed and ETSL is much smaller. As a result, the car velocity can temporarily exceed the ETSL pattern limit during a normal NTSD backup pattern slowdown, which would activate the ETSL system and shut the car down. To avoid interference between the NTSD and ETSL systems, the margin between the NTSD and normal system must be kept sufficiently small. However, this is difficult to do without causing nuisance clamping of the normal slowdown pattern by the NTSD pattern.
Second, as shown in FIG. 3, since the NTSD pattern is a time based integrator with a fixed rate of change, if the NTSD system has too few hoistway vanes for a proper setup, the setup attempt produces a learned pattern that has too large a top NTSD step, resulting in an NTSD that cannot "catch" the subsequent steps. Thus, as shown in FIG. 3, when the car passes the first vane V.sub.1, the NTSD speed begins to decrease at the specified deceleration rate. However, the NTSD speed for the next vane V.sub.2 is so much less than V.sub.1 that, when the car reaches vane V.sub.2, the NTSD speed has not yet decreased to the V.sub.2 velocity. A car that follows such an NTSD pattern will therefore be travelling well above normal speed for most of the slowdown, and is likely overshoot the terminal landing and reach the final limit switch, which shuts down the car.
Third, as shown in FIG. 4, where the terminal vane placement is not ideal for the given elevator speed and deceleration rate, the NTSD backup pattern will clip the normal pattern during the jerk into deceleration. As shown in FIG. 4, as the car passes vane V.sub.1, the NTSD speed follows a constant deceleration rate, until it reaches the V.sub.2 speed, whereupon it remains at the V.sub.2 speed until reaching vane V.sub.2. However, vanes V.sub.1 and V.sub.2, which are located in the region where the car jerks into deceleration, are too far apart. The result is that the NTSD speed value is lower than normal car speed during part of the elevator travel between vanes, resulting in unwanted clipping of the normal slowdown pattern.
Fourth, the NTSD curve is calculated assuming a normal travel time between two vanes during a high speed run. However, where the elevator executes a one-floor run, at the point where the car jerks into deceleration, it is not travelling at rated speed, and the travel time between vanes is greater than normal. As shown in FIG. 5, this means that the NTSD speed decreases to the speed for the next vane before the car has actually reached the next vane and, as in the case of FIG. 4, the NTSD deceleration profile is partly a stepped curve. As the car jerks into deceleration mode, the car is decelerating at a deceleration rate less than the NTSD curve. On certain speed and deceleration rate combinations, the two patterns converge, causing an unwanted NTSD clamping of the normal pattern.
Therefore, much trial-and-error work may be required to make the existing NTSD system work around these problems, thus increasing installation and servicing costs.